Bronze materials such as Cu--Sn--Pb and lead-bronze materials are commonly used as materials for copper-base sintered bearings. Such materials are often integrally bonded to an iron-base metal backing to form a double layered sintered contact element such as a track roller provided in the base carrier of a construction machine.
As bearings used under high surface pressure and low speed conditions such as bushings for use in a construction machine, carburized or high-frequency-hardened wear-resistant steel bushings are widely known. When used in a construction machine, a bushing is subjected to high surface pressure and severe lubricating conditions and likely to generate undesirable abnormal noise during operation. To cope with this problem, there are often used high strength brass bushings or the above-described steel bushings which have undergone lubricating film coating treatment. High strength brass bushings are especially attractive because of their superior conformability. To prolong the intervals at which grease is fed to a bearing disposed in a construction machine etc., there has been proposed a graphite-embedded bearing material (e.g., 500SP produced by OILLESS INC.) that is produced by machining a high strength brass bushing to have holes and embedding porous graphite in the holes to be impregnated with oil. For the same purpose, a sintered metal body containing a large amount of solid lubricant (e.g., the SL alloys produced by TOSHIBA TUNGALOY CO., LTD.) is used.
Japanese Patent Publication (KOKOAI) No. 5-156388 (Japanese Patent Publication (KOKOKU) No. 56-12288) discloses a double layered sintered contact element for use under high surface pressure and its producing method. This contact element is formed by integrally bonding an aluminum-bronze-base sintered contact alloy, in which graphite is dispersed as a solid lubricating component within the range of 3 to 8 wt %, to a steel plate through a bonding layer.
Japanese Patent Publication (KOKAI) No. 3-232905 discloses a double layered sintered contact element having superior resistance to high load and impact. This element comprises (i) a metal backing that is made of a steel plate having a plurality of independent protrusions at its surface or steel plate having a series of connected protrusions and a plurality of independent recesses defined by the respective protrusions and (ii) a copper-base sintered alloy layer that is integrally formed with the metal backing so as to cover the protrusions of the surface and in which graphite is dispersed as a lubricating component in an amount of at least 3 wt %. This copper-base sintered alloy layer is composed of a low-density, high-oil-impregnating alloy region and a high-density, low-oil-impregnating alloy region.
Double layered sintered contact elements having a lead- bronze-base or bronze-base sintered contact layer are versatilely used as a sliding contact member of good conformability for the aforesaid lower track roller or engine metal, under low load, high speed and well lubricating conditions. However, they are inadequate for use as a bushing for a construction machine since they are easily worn out or fatigued due to lack of hardness for withstanding the high surface pressure and severe lubricating conditions.
When a steel bushing is used as a bushing in a construction machine, it is not fatigued but likely to cause seizure or unpleasant abnormal noise.
High strength brass contact materials produced by casting are substantially free from fatigue and generate little noise, but easily become short of lubricity. Therefore, they cannot completely prevent noise generation and are unavoidably susceptible to rapid wear when they are used in extremely low speed and high load applications such as when adapted in a construction machine.
Generally, in a high strength brass base contact material formed as a sinter contact alloy, the steam pressure of Zn contained is too high to carry out high density sintering. Therefore, sintering must be carried out under a pressurized condition when producing a double layered sintered contact element from such high strength brass contact material and, in consequence, production cost cannot be reduced.
In the case of steel bushings coated with a lubricating film, formation of a thick lubricating film is generally difficult and expensive and it is therefore apparent that after the lubricating film provided for such bushings is worn out, abnormal noise and seizure occur, like the case of ordinary steel bushings.
The graphite-embedded high strength brass bushing having more self-lubricity (e.g., 500SP produced by OILLESS INC.) requires a punching process for making a large number of holes for graphite embedding as well as a graphite filling process, in order to achieve high self-lubricity, which significantly increases cost. In many cases, the ratio of holes for graphite is normally restricted to 25 to 30% on a basis of area in view of cost, and therefore they cannot provide satisfactory self-lubricity over a long period of time.
The degree of sintering is a problem for a sintered metal body containing a large amount of known solid lubricant such as graphite, BN and MoS.sub.2 and therefore a pressure sintering means such as a hot press is required in order to achieve high density, resulting in increased cost. In addition, since the conventionally used solid lubricant is extremely soft, even a high-density metal sintered body suffers from high brittleness.
In the case of the double layered sintered contact element comprising (i) a steel plate serving as a metal backing and having a plurality of independent protrusions or a series of connected protrusions on its surface and (ii) an integral copper-base sintered contact alloy layer containing at least 3 wt % graphite, the steel plate backing formed into a specified shape is expensive and it is extremely difficult and costly to manufacture a metal backing which has protrusions (high-density portions) for sustaining a bearing load, in an area ratio of 30% or more.
Additionally, when bending the above material to produce a tubular bushing, bending stress is concentrated on the protrusions of the steel plate which are harder than other areas, so that exfoliation is likely to occur in the joint interface between the steel plate and the copper-base sintered contact material and the deformation resistance occurring in the bending operation is unevenly distributed in the steel plate. This makes it difficult to uniformly bend the material into a tubular shape. In consequence, a lot of machining work is involved in producing a tubular bushing, and the cost for inspecting the quality of bonding between the contact material and metal backing of the final product increases.
The invention is directed to overcoming the foregoing problems. Therefore, an object of the invention is to provide copper-base and/or iron-base contact materials in which Pb intermetallic compounds are dispersedly precipitated to achieve highly improved sliding properties. Another object of the invention is to provide double layered contact elements and their economical producing methods, the contact elements being produced by sinter-bonding the above contact materials to an iron-base metal backing in an integral fashion and being improved in oil impregnation and lubricity.